AP CSA 1.12：对象：类的实例
In earlier lessons, we used classes such as Turtle, World, and Math. In this lesson, we focus on what classes and objects mean in Java.
前几课我们已经用过 Turtle、World 和 Math 这样的类。这一课，我们重点理解 Java 中的类和对象到底是什么意思。
A class is a blueprint; an object is a specific instance created from that blueprint. 类是蓝图；对象是根据这个蓝图创建出来的具体实例。
Core Concepts:
核心知识点
1. Main Idea
核心理解
Java is an object-oriented programming language. Objects combine data and methods into one unit.
Java 是一门面向对象的编程语言。对象把数据和方法组合在一起。
A class defines what kind of object can be created.
类定义可以创建什么样的对象。
An object is a specific instance created from a class.
对象是根据类创建出来的具体实例。
| Term | Meaning | 中文理解 |
|---|---|---|
| Class | blueprint / type | 蓝图 / 类型 |
| Object | instance of a class | 类的具体实例 |
| Attribute | data stored about an object | 对象保存的数据 |
| Behavior | action an object can perform | 对象能执行的动作 |
2. Class vs. Object
类 vs. 对象
A class is like a blueprint for a house.
类（class） 就像房子的蓝图。
An object is like an actual house built from that blueprint.
对象（object） 就像根据蓝图建出来的一栋具体房子。
Another analogy:
另一个类比：
Class = cookie cutter
Object = cookie
A cookie cutter can make many cookies. A class can create many objects.
一个饼干模具可以做很多块饼干。一个类也可以创建很多个对象。
Example:
Turtle yertle = new Turtle(habitat);
Turtle myrtle = new Turtle(habitat);
Here, Turtle is the class. yertle and myrtle are variables that refer to two different Turtle objects.
这里 Turtle 是类；yertle 和 myrtle 是两个变量，分别指向两个不同的 Turtle 对象。
3. A Class Defines a New Type
类定义一种新的类型
A class defines a new reference type.
类定义一种新的引用类型。
Just like int can be used to declare integer variables:
就像 int 可以用来声明整数变量：
int score = 95;
A class name can be used to declare object variables:
类名也可以用来声明对象变量：
Turtle yertle;
But this only declares the variable. To create the actual object, use new.
但这只是声明变量。要真正创建对象，需要使用 new。
Turtle yertle = new Turtle(habitat);
4. Attributes and Behaviors
属性与行为
A class defines what its objects have and what they can do.
类定义对象“有什么”和“能做什么”。
| Concept | Meaning | Turtle Example |
|---|---|---|
| Attribute | data about the object | color, position, heading |
| Behavior | action the object can perform | forward(), turnLeft(), turnRight() |
Attributes are often nouns or adjectives.
属性通常像名词或形容词。
Examples:
color
height
width
xPos
yPos
heading
Behaviors are often verbs and are implemented as methods.
行为通常像动词，并且通过方法实现。
Examples:
forward()
turnLeft()
turnRight()
backward()
For AP-style questions, use this simple test:
AP 题里可以这样判断：
Is it data? → attribute
Is it action? → behavior / method
5. Objects Have Their Own Attribute Values
对象有自己的属性值
Objects created from the same class share the same kind of attributes and behaviors.
同一个类创建出来的对象，拥有同一类属性和行为。
But each object can have different values for those attributes.
但每个对象的属性值可以不同。
For example, two Turtle objects may both have a color and a position, but their actual colors and positions can be different.
例如，两个 Turtle 对象都可以有颜色和位置，但它们具体的颜色和位置可以不一样。
Turtle yertle = new Turtle(habitat);
Turtle myrtle = new Turtle(habitat);
yertle.forward();
myrtle.turnRight();
myrtle.forward();
yertle and myrtle are separate objects. Calling a method on one object does not automatically call it on the other.
yertle 和 myrtle 是两个独立对象。对其中一个对象调用方法，不会自动影响另一个对象。
6. Reference Variables
引用变量
A variable of a primitive type stores a simple value.
基本类型变量直接存储一个简单值。
int age = 7;
A variable of a class type stores an object reference.
类类型变量存储的是对象引用。
Turtle yertle = new Turtle(habitat);
For AP CSA 1.12, you can think of an object reference as the memory address of the object.
在 AP CSA 1.12 中，可以把对象引用理解成对象在内存中的地址。
| Variable Type | Stores |
|---|---|
int, double, boolean |
actual primitive value |
class type, such as Turtle |
reference to an object |
This is why class types are called reference types.
这就是为什么类类型被称为 引用类型（reference types）。
7. Creating and Using Objects
创建和使用对象
The general pattern for creating an object is:
创建对象的一般格式是：
ClassName variableName = new ClassName(arguments);
Example:
World habitat = new World(300, 300);
Turtle yertle = new Turtle(habitat);
Then use the dot operator to ask the object to do something.
然后使用点运算符让对象执行动作。
yertle.forward();
yertle.turnLeft();
yertle.forward();
The general method-call pattern is:
方法调用的一般格式是：
objectName.methodName(arguments);
If there are no arguments, the parentheses are still required.
如果没有实参，括号仍然必须写。
yertle.forward();
Some methods can take arguments.
有些方法可以接收实参。
yertle.forward(50);
This asks yertle to move forward 50 pixels.
这表示让 yertle 前进 50 像素。
8. Small Turtle Example
Turtle 小例子
import java.awt.*;
import java.util.*;
public class TurtleTest
{
public static void main(String[] args)
{
World habitat = new World(300, 300);
Turtle yertle = new Turtle(habitat);
yertle.forward();
yertle.turnLeft();
yertle.forward();
habitat.show(true);
}
}
What to notice:
需要注意：
-
Worldis a class. -
habitatrefers to aWorldobject. -
Turtleis a class. -
yertlerefers to aTurtleobject. -
forward()andturnLeft()are methods. -
yertle.forward();calls a method on theyertleobject. -
World是类。 -
habitat指向一个World对象。 -
Turtle是类。 -
yertle指向一个Turtle对象。 -
forward()和turnLeft()是方法。 -
yertle.forward();是对yertle对象调用方法。
9. Inheritance: Know the Idea Only
继承：只需理解概念
Inheritance means one class can be based on another class.
继承（inheritance）指的是一个类可以基于另一个类。
For example, Dog, Cat, and Turtle could all be subclasses of a more general Pet class.
例如，Dog、Cat 和 Turtle 都可以是更通用的 Pet 类的子类。
Important AP CSA note:
AP CSA 重点提醒：
Designing and implementing inheritance relationships are outside the current AP CSA exam scope.
也就是说：
设计和实现继承关系不属于当前 AP CSA 考试范围。
So for this lesson, just know the vocabulary:
所以这一课只需要知道基本词汇：
| Term | Meaning |
|---|---|
| superclass | more general class |
| subclass | more specific class based on a superclass |
| Object class | all Java classes are subclasses of Object |
Do not spend too much time designing inheritance trees for the exam.
备考时不要花太多时间设计继承结构。
10. Common Beginner Mistakes
常见初学者错误
| Mistake | Wrong Idea / Code | Why Wrong | Correct Understanding | 中文解释 |
|---|---|---|---|---|
| Confusing class and object | saying Turtle is an object |
Turtle is the class/type |
yertle refers to a Turtle object |
类和对象要分清 |
| Thinking a class can only make one object | “One class creates one object only” | A class can create many objects | create as many as needed | 一个类可以创建很多对象 |
| Confusing attributes and methods | saying forward() is an attribute |
forward() is an action |
it is a method / behavior | 方法是行为，不是属性 |
Forgetting new |
Turtle yertle; yertle.forward(); |
object may not have been created | Turtle yertle = new Turtle(habitat); |
声明变量不等于创建对象 |
| Calling method without object | forward(); |
forward belongs to a Turtle object |
yertle.forward(); |
对象方法要通过对象调用 |
| Forgetting parentheses | yertle.forward; |
method calls need () |
yertle.forward(); |
调用方法必须写括号 |
11. Debugging Example
调试例子
Buggy code:
World habitat = new World(300, 300);
Turtle yertle;
yertle.forward();
habitat.show(true);
Problem:
yertle was declared, but no Turtle object was created.
问题：
yertle 被声明了，但还没有真正创建 Turtle 对象。
Fixed code:
World habitat = new World(300, 300);
Turtle yertle = new Turtle(habitat);
yertle.forward();
habitat.show(true);
| Bug | Type | Fix |
|---|---|---|
declaring Turtle yertle; but not creating an object |
compile-time / object creation issue | use new Turtle(habitat) |
yertle.forward; |
compile-time error | use yertle.forward(); |
confusing Turtle with yertle |
concept error | Turtle is the class; yertle refers to an object |
12. Mini Practice
小练习
Practice 1: Choose the Best Answer
练习 1：选择最佳答案
A student has created a Dog class. The class contains:
breed
age
name
The object pet is declared as type Dog.
Which description is accurate?
A. An attribute of the name object is String.
B. An attribute of the pet object is name.
C. An instance of the pet class is Dog.
D. An attribute of the Dog instance is pet.
Answer:
B. An attribute of the pet object is name.
Explanation: pet is an object of type Dog, and name is one of its attributes.
解释：pet 是 Dog 类型的对象，而 name 是它的一个属性。
Practice 2: Attribute or Behavior?
练习 2：属性还是行为？
For a Turtle object, classify each item:
color
heading
forward()
turnRight()
Answer:
Attributes: color, heading
Behaviors / methods: forward(), turnRight()
Explanation: attributes are data; methods are actions.
解释：属性是数据；方法是动作。
Practice 3: Predict the Role
练习 3：判断角色
In the code below, what is Turtle and what is yertle?
Turtle yertle = new Turtle(habitat);
Answer:
Turtle is the class.
yertle is a variable that refers to a Turtle object.
解释：
Turtle 是类。
yertle 是一个变量，指向 Turtle 对象。
Practice 4: Fix the Code
练习 4：修复代码
Fix the code so the Turtle object is created before it moves.
World habitat = new World(300, 300);
Turtle yertle;
yertle.forward();
habitat.show(true);
Fixed code:
World habitat = new World(300, 300);
Turtle yertle = new Turtle(habitat);
yertle.forward();
habitat.show(true);
Explanation: declaring an object variable is not the same as creating the object.
解释：声明对象变量不等于创建对象。
Quick Checklist
快速检查清单
Before answering an objects question, check:
做对象相关题目前，检查：
- Is this asking about a class or an object?
- Is the class the blueprint/type?
- Is the object a specific instance of the class?
- Can one class create many objects?
- Is the item data or an action?
- If it is data, is it an attribute / instance variable?
- If it is an action, is it a method / behavior?
- Does the object variable store a reference?
- Was the object actually created with
new? - Is the dot operator used with the correct object?
- Does the method call include parentheses?
- Is the inheritance detail only vocabulary, not something you need to design?